Kopycinski's Brain: Jeep Cherokee/Comanche 4.0L Tips

The Jeep XJ Cherokee has been a very popular SUV, and there are plenty of high-mileage ones running around these days.

You've also seen us here at OFF-ROAD build several XJs, because they’re an affordable 4WD. But, as they age, parts deteriorate, and fail. This month we’ll give you a quick run-down of four common problems and potential cures for the ailments on 4.0L I-6 Cherokees and Comanche pickups.

The Jeep XJ has come equipped with open and closed cooling systems over the years. The closed systems have no radiator fill cap. Coolant is added to a catch can that is mounted near the passenger side firewall. Whenever the coolant system is opened, such as when a hose is removed, air enters. It can then be difficult to purge the air back out of the system and you may experience erratic temp gauge fluctuations and overheating behavior. Filling the system through the upper radiator hose is best. Additionally, drilling 1/8-inch holes at the 6:00 and 12:00 positions in the thermostat will aid with purging. As a final resort, park the vehicle slightly nose down and allow the air to burp from the system through the hole at the back of the engine head where the temp sensor threads in.

These 4.0L vehicles are known to run on the warm side. When you add bigger tires and are pushing more weight, the engine temps can rise above a comfortable level. For situations where air stagnation under the hood may be a problem, the addition of a set of hood louvers can help vent some of that hot air. We’ve seen wheelers adapt salvaged hood and fender vents off passenger cars, and aftermarket louver panels are available as well.

AMC used the Renix system on Jeep Cherokee and Comanche models from 1987 to 1990. It’s an intelligent electronic ignition combined with multiport fuel injection. It can be a fairly reliable engine management system, but has a few common failing points as it starts to age. One is related to wiring harness grounds that grow resistive and no longer make solid electrical contact to the body ground point. These can cause erratic or excessively high idle speed. Running a new ground wire to the firewall from these two throttle body connectors bypassed some of the aging harness connections and restore a smooth idle.

The throttle position sensor (TPS) on these engines can fail over time and cause uneven throttle response. More notably, failure can cause an AW4 auto transmission to remain stuck in first gear. The TPS pictured here failed and was later opened up. Mechanical wear had degraded the flexible circuit card that is curved inside the housing. In this case, the vehicle shifted fine when cold, but stopped shifting once the TPS got hot in the engine compartment.